Antarctic
Diving 8 - Flakes of Ice Forming at 5m

When the temperature drops
in the sea, sometimes in the shallows ice starts to form below
the surface as it is here. It starts as a nucleation point
which can happen on submerged sea weed as here as delicate flakes
of ice randomly forming ice crystals. In the shallows of 1-2m,
ice may form on the sea bed from below as above.

Normally ice freezes from the
surface of the water down as ice has a very unusual property
amongst all substances in that as it turns from liquid to a
solid, so it expands rather than contacts which is normal for
cooling materials. This is why sea-ice floats rather than sinks
as it would if it was almost anything else. The effect on the
planet is profound, if water behaved "normally" then the oceans
would be mainly solid ice other than the upper reaches and warmer
climates.

Water has its maximum density
at 4C and so all of the depths of the worlds oceans are at or
around 4C as this temperature water descends due to it being
denser than water around it of other temperatures.